What Is Camera Exposure?

July 8, 2015

I’ve been studying photography for quite a while now and I can’t tell you how many times I refer back to the basics to reassess my shooting style and capabilities. It seems that each time I do this, something new comes to mind that I can take advantage of. And the primary area of “basics” I look back to is exposure. When it comes to photography, there are few subjects more critical than this one. That’s why I decided to write a post that explores it in detail.

In this post, I’m going to discuss the three primary elements that make up, or affect, exposure. These three elements, in some circles, are called the “Exposure Triangle.” They are aperture, shutter speed and ISO.

What Is Exposure?

Most simply put, exposure, when relating to photography, is the amount of light that makes it through the various camera apparatus to the image sensor. If an amount of light that’s let through is too great for your photo’s intent, your photo will be “whitish” or washed out (overexposed). If the amount of light that’s let through is too little for your photo’s intent, your photo will be dim and dark (underexposed).

When folks discuss camera exposure, they often enjoy using examples to help explain what things are about. So far in my journey, I’ve heard two that makes sense to me. I’ll go over both of them here. The examples have to do with the human eye and a rain bucket.

The Human Eye

The function of the human eye (and most other eyes for that matter) can be quite easily compared to the function of how a camera works. Within and around the eye, there’s an iris, an eyelid and an overall sensitivity.

The iris of the eye is quite similar to a lens aperture, in that the both shrink and grow in relation to the amount of light that’s allowed to pass through to a sensor. The eyelid can be compared to the camera’s shutter. Both the eyelid and the shutter open and close with the intent to control the passage of light. And finally, both the eye and the camera have a sensitivity that is inherent to them. The eye uses a retina to adjust for this sensitivity and a camera uses ISO. Both move, or can move, and change based on available light and the photographic intent.

The Rain Bucket

The rain bucket example is a fun one because it’s really easy to understand. It’s a bit longer to explain than the eye example, but it’s worth it.

Let’s pretend it’s raining outside. The rain isn’t coming down too hard – just hard enough for a steady fall. When using the rain bucket example, we can compare the volume of rain that’s falling from the sky to the ISO setting we utilize on our camera. Sometimes there is all the rain in the world and sometimes there’s not much rain at all. Again, in this case, we’ll say the amount of rain falling is medium.

Let’s also say that we’ve got a bucket that we can place outside to catch some rain. The opening at the top of the bucket can be compared to the aperture of a camera lens. If the bucket has a big opening, a large amount of rain is let through, therefore filling the bucket faster. If the aperture on a camera is large, a lot of light is let through.

Now, if we only wanted to catch a certain amount of rain water, it would be silly for us to leave the bucket out in the rain all day. Depending on how much rain we wanted, we would limit the length of time we leave the bucket outside. This can be compared to the shutter speed in a camera. Just as if we quickly place the bucket outside that then pull it back in really fast, we can set the camera to quickly open its shutter and then close it just as quickly, limiting the amount of light that’s allowed to pass.

I’m going to give a scenario to better illustrate what I’m talking about. Say we wanted to catch one gallon of rain water. If the rain is still falling at a medium rate (ISO), we have two settings we can adjust to meet out goal. We can either use a bucket with a narrower opening (aperture) and leave it out in the rain for a longer period of time (shutter speed) or we can use a bucket with a really wide opening (aperture) and leave it outside for a shorter amount of time (shutter speed). If the rain begins falling harder, we can adjust both of the variables to compensate for the heavier downfall.

As you can see, the three settings work together and offer a variety of flexibility. Since, oftentimes, environmental variables are out of our control, we set our cameras to make the best use of what we’re facing.

In this post, I’m going to cover some specifics on how aperture, shutter speed and ISO affect camera exposure. In later posts, I’m going to discuss the related characteristics of each of these areas and how they can increase a photographer’s creative license.

Shutter Speed

If we use another example to explain shutter speed, we can do it like this: close your eyes. Now open them very quickly and close them again. At first, all you saw was darkness. For that split second, you saw light and then when you closed them again, you saw darkness. You probably remember what you saw when you opened your eyes. Consider that a photo.

In a camera, a shutter is the mechanism that covers the sensor. When it opens, it allows the sensor to “see” light. Shutter speed is the measurement that’s used to determine how long the sensors sees that light. The longer the shutter is open, the more light will pass through to the sensor. The more light, the brighter the image.

Shutter speed is measured in seconds and since most photos are taken in normal daylight, the speed will generally be less than a second, or a fraction of one.

Below, I’ve created a handy list of when you might use a specific shutter speed. Please take a look at it to get an idea of how fast a shutter might move under a particular circumstance. All measurements are in seconds.

1/4000 – 1/1000: Use this speed when you are photographing extremely fast action that’s very close up.

1/500 – 1/250: You may want to use this speed for every day action shots, such as sports or wildlife. Also, you can use this speed for hand held photography as well as when using telephoto lenses.

1/100 – 1/50: This is an every day speed that works well when hand holding your camera without a zoom lens.

1/30 to 1/2: Use this speed to add motion blur to your photos.

1/2 – 2: You can use this speed to take still shots with movement in one portion of it, such as with rivers and waterfalls, for a “silk” effect. You’ll need a tripod for this type of photography.

1 – 30+ seconds: You must use a tripod for this speed. It’s primarily used for night and low light photography.

Aperture

As part of your camera lens, the aperture acts just like the iris of your eye. The difference is, the aperture is constructed of metal blades that are controlled by either you or your camera’s automatic settings. The aperture opens and closes, much like a flower’s petals do during the day and during the night.

Aperture directly controls how much light passes through the lens into the camera. It’s the size of the hole that does this – the larger the hole, the more light. The smaller the hole, the less light.

Sort of like shutter speed fractional values, aperture size is specified in terms of f-stop values. This is where people get confused. The smaller the f-stop value number, the larger the aperture and the larger the f-stop value number, the smaller the aperture. Now, notice how I said “f-stop value number” and not just “f-stop value.” That’s because f-stop is a ratio. It’s written like this:

f/22

This refers to the ratio of the size of the aperture to the focal length of the lens.

I’ve been told that a good way to remember this ratio is that a larger number stops more light, resulting in a smaller hole. A smaller number stops less light, resulting in a larger hole.

Photographers like to use a lot of lingo. For instance, when a photographer says they are “opening up the lens,” they are actually increasing the size of the aperture (lower number). Similarly, they may say they are “stopping down,” which means the same thing.

When looking at actual f-stops, you can view full stops, half stops or third stops. I’ll give you some examples of what full stops look like:

f/1.4f/2f/2.8f/4f/5.6f/8f/11f/16f/22

ISO

ISO determines how sensitive the camera’s sensor will be to the incoming light that both the shutter speed and aperture control. Years ago, photographers specified their ISO values by the film they placed in their cameras. Now, since film is rarely used in the general population, a photographer can control their ISO value simply by turning a dial on their camera.

I’ll talk more about ISO values in later posts, but I want to mention that, in general, it’s more desirable to utilize a lower ISO value. As the ISO value increases, digital noise increases in the resulting photo. So, unless there’s a reason for it, keep the ISO value as low as possible on your camera. The good news is, camera manufacturers have come a long way in reducing much of this noise, so photographers will soon have the luxury of utilizing much higher ISO values, with clean results, than ever before.

Before I close, I wanted to tell you that common ISO values include 100, 200, 400 and 800, with higher and lower values populating both sides of this spectrum. Those higher and lower values are generally for specialty photography though.

For a more in depth conversation regarding the three sides of the exposure triangle, please take a look at these posts:

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I hope I helped you understand some basic camera exposure theory. If you have any comments or questions, please leave them below. Also, if you’re interested in more photography tips, please check out the photography category up above, at the top of the page.

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About Jay Gaulard

Hi. My name is Jay Gaulard and I've been designing websites and taking photographs since 2002. My passions lie with learning the latest coding techniques and development tools as well as staying on top of the digital imaging world. When I'm not studying or writing, I'm practicing Brazilian Jiu-Jitsu in Maine, USA.

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